Any ophthalmic lens intended, to be worn in a frame is associated with wearer data and/or spectacle frame data. The ophthalmic prescription is one piece of wearer data; it may comprise a power prescription, plus or minus, as well as an astigmatism prescription. These prescriptions correspond to corrections enabling the lens wearer to correct vision defects. A lens is fitted in the frame in accordance with the prescription and the position of the wearer's eyes relative to the frame. For presbyopic wearers, the value of the power correction is different for far vision and near vision, due to the loss of accommodation in near vision. The prescription thus consists of a far-vision power value and an addition (or power progression) representing the power increment between far vision and near vision; basically this means a far-vision power prescription and a near-vision power prescription. Other wearer data may be taken into account, such as prismatic prescriptions, the position of the center of rotation of the eye, or other data. The frame data may, for example, include pantoscopic tilt, face form wrap, dimensions, templates, or other data.
Such an ophthalmic lens can be made by directly machining one of both faces of a glass block, using appropriate equipment. An ophthalmic lens may also be made from a semi-finished lens blank. A semi-finished lens blank has a defined face with a specific surface geometry, and an unfinished face which allows adapting the lens to the wearer data and/or data for the frame chosen by the wearer. An ophthalmic lens can then be made from a semifinished lens blank simply by machining the unfinished face, which is generally a spherical or toroidal surface.
A product line is defined as a family of lenses having common characteristics. For each line, a subset of wearer data and/or frame data are defined which determine the characteristics of the defined faces of the semifinished lens blanks. A set of semi-finished lens blanks is thus defined. Each semi-finished lens blank in a line must be able to satisfy a subset of wearer data and/or frame data. Each semi-finished lens blank must have the geometric characteristics, particularly the thickness, to allow machining the face opposite the defined face and allow making all the lenses in the family corresponding to the subset of wearer data and/or frame data for which it was defined.
For example, a line having a given progressive design with 5 base values, 12 addition values, and 2 eyes means 120 faces to accommodate. One could also define a line with other data, in particular design data (soft or hard for example), sphere values, cylinder values, data concerning the eye (right eye or left eye or customization data for the eye of the wearer for example). For a given line, the number of semi-finished lens blanks to be prepared is determined by the number of faces to be accommodated to cover all the wearer data and/or frame data that are possible with this line product. For example, 5 to 6 semi-finished lens blanks are necessary to cover unifocal prescriptions within a range of −8 to +6 diopter for the sphere component and 0 to 4 diopter for the cylinder component.
The use of semi-finished lens blanks is common in the field of ophthalmic lenses, because it is then possible to store a limited number of items while still satisfying customer needs across the entire product line. In fact, the same semi-finished lens blank reference, meaning a lens blank having a face of a certain geometry, can be used to produce an assortment of lenses satisfying a subset of wearer data and/or frame data.